There are a number of assays and sensors for the detection of the presence and/or concentration of specific substances in fluids and gases. Many of these rely on specific ligand/antiligand reactions as the mechanism of detection. That is, pairs of substances (i.e. the binding pairs or ligand/antiligands) are known to bind to each other, while binding little or not at all to other substances. This has been the focus of a number of techniques that utilize these binding pairs for the detection of the complexes. These generally are done by labelling one component of the complex in some way, so as to make the entire complex detectable, using, for example, radioisotopes, fluorescent and other optically active molecules, enzymes, etc.
Other assays rely on electronic signals for detection. Of particular interest are biosensors. At least two types of biosensors are known; enzyme-based or metabolic biosensors and binding or bioaffinity sensors. See for example U.S. Pat. Nos. 4,713,347; 5,192,507; 4,920,047; 3,873,267; and references disclosed therein. While some of these known sensors use alternating current (AC) techniques, these techniques are generally limited to the detection of differences in bulk (or dielectric) impedance.
The use of electrophoresis in microfluidic methods to facilitate the binding of biological molecules to their binding partners for subsequent detection is known; see for example U.S. Pat. Nos. 5,605,662 and 5,632,957, and references disclosed therein.
Similarly, electronic detection of nucleic acids using electrodes is also known; see for example U.S. Pat. Nos. 5,591,578; 5,824,473; 5,705,348; 5,780,234 and 5,770,369; U.S. Ser. No. 08/911,589; and WO 98/20162; PCT/US98/12430; PCT/US98/12082; PCT/US99/10104; PCT/US99/01705, and PCT/US99/01703.
One of the significant hurdles in biosensor applications is the rate at which the target analyte binds to the surface for detection and the affinity for the surface. There are a number of techniques that have been developed in nucleic acid applications to either accelerate the rate of binding, or to concentrate the sample at the detection surface. These include precipitation of nucleic acids (see EP 0 229 442 A1, including the addition of detergents (see Pontius et al., PNAS USA 88:8237 (1991)); partitioning of nucleic acids in liquid two phase systems (see Albertsson et al., Biochimica et Biophysica Acta 103:112 (1965), Kohne et al., Biochem. 16(24):5329 (1977), and Müller, Partitioning of Nucleic Acids, Ch. 7 in Partitioning in Aqueous Two-Phase Systems, Academic Press, 1985)), as well as partitioning in the presence of macroligands (see Müller et al., Anal. Biochem. 118:269 (1981)); and the addition of nucleic acid binding proteins (see Pontius et al., PNAS USA 87:8403 (1990) and U.S. Pat. No. 5,015,569), all of which are expressly incorporated by reference. In addition, partitioning systems for some proteins have also been developed, see Gineitis et al., Anal. Biochem. 139:400 (1984), also incorporated by reference.
However, there is a need for a system that combines acceleration of binding of target analytes, including nucleic acids, to a detection electrode for subsequent electronic detection.